This invention relates to a method of determining whether to compress data transmitted from a digital private branch exchange (hereinafter, PBX) to a digital communication line, and whether to decompress data transmitted from the digital communication line to the digital PBX. The invented method is suitable for use, for example, by a multimedia multiplexer in transmitting PCM audio data.
Large corporations and other entities with sites at diverse locations often construct in-house communication networks comprising nodes linked via high-speed digital communication lines leased from a common carrier. Switching at each node is performed by a digital PBX.
Increasingly, such networks are used for multimedia purposes, carrying both telephone conversations and other types of information such as video images and computer data. The PBXs are then connected to the leased communication lines through multimedia multiplexers. To enable the leased lines to carry more information, a multimedia multiplexer usually compresses audio and video data when sending such data on the line, the data being decompressed by the multimedia multiplexer at the receiving end of the line.
To avoid excessive distortion, it is preferable that the compression and decompression processes be performed only at the originating and destination nodes, not at any intermediate nodes through which the data may be routed. Since a PBX is capable of determining whether it is at an originating, destination, or intermediate node, a prior-art scheme was to have each PBX notify its connected multimedia multiplexers whether compression and decompression were required or not.
Methods of passing compress and non-compress commands from a PBX to a multimedia multiplexer have not been standardized, however, so when the prior-art scheme is adopted, it restricts the selection of PBX equipment. That is, the PBXs must have a non-standard feature for sending such commands, and must use particular commands understood by the multimedia multiplexers. In addition to limiting the range of selection, these restrictions tend to raise the cost of building a network by preventing the use of inexpensive PBX equipment without such special features.